Cycloolefin random copolymers composed of ethylene and specific bulky cycloolefins are synthetic resins which are well balanced among optical quality, mechanical properties and thermal properties, and are used in the field of optical materials such as optical memory disc and optical fiber.
When an optical memory disc is prepared from such cycloolefin random copolymer as mentioned above, however, the disc, even on rare occasions, commits an error in reading. The present inventors extensively researched with the view of minimizing an error in reading of the optical memory disc prepared from the cycloolefin random copolymer, and eventually have found that polyethylene formed as a by-product during the copolymerization and contained as an impurity in said cycloolefin random copolymer causes in part this error in reading. The present invention has thus been accomplished on the basis of this finding.
On the one hand, it is strongly desired that a process for preparing cycloolefin random copolymers is developed to make their appearance, by which the content of polyethylene contained as an impurity in the resulting copolymers can be decreased.
In this connection, the cycloolefin random copolymers referred to above have heretofore been prepared by copolymerization of ethylene and cycloolefin in the presence of a catalyst formed from a soluble vanadium compound and an organoaluminum compound using hydrocarbon such as cyclohexane or toluene, or the above-mentioned cycloolefin as a solvent. The present inventors extensively prosecuted research with the view of inventing a process for preparing cycloolefin random copolymers which have been reduced in content of polyethylene contained as an impurity and which are excellent in transparency, and eventually have found that the desired process can be attained by copolymerizing ethylene and cycloolefin under specific conditions or subjecting the copolymer solution obtained to specific operation.